Question: Would it really have been possible for Nygma to modify the security footage to make it look as if Stickley was committing suicide at the time, with Nygma nowhere in sight?
Question: If Bruce had backed Edward's invention, would Edward have still done what he did in the movie (using the device to grow smarter, try to outdo Bruce etc)? Or did Bruce spurning him cause him to do that?
Answer: And more precisely, narcissists like Edward Nygma/The Riddler can barely tolerate when someone else refuses to have their ideas, viewpoints, or feelings prioritised or contrasted against others, even if it is for a legitimate cause or logical reasoning. The fact that Nygma already had a sense of envy towards Bruce's fame that he idealised would all but guarantee that he would seek some kind of revenge or humiliation on Bruce, no matter what the circumstances would be if Bruce had in fact done so later.
Answer: It's hard to say for certain. If Bruce had approved of the project, there would have been a lot more volunteers and oversight of its production, which would have alerted Bruce of the device's side effects, at which point he or one of his underlings would order its production to cease. Edward would still feel spurned and motivated to continue his work, but without the resources of Wayne Industries at his disposal, he may have a more difficult time seeing his plan through.
Answer: That's an extremely difficult question to answer because you have to look at it from two perspectives. In real life at the time in the 90s? Probably not, because the technology wasn't really there to convincingly create a convincing computer simulation/fake footage that quickly. (Even the special effects wizards who had months to make the movie did a good job but didn't quite nail the digital Batman in the few shots he's CGI.) But in the context of the movie? Yes. You have to remember, this movie is set in a highly stylized, fictionalized universe. One with superheroes, supervillains, highly advanced technology, doomsday machines, and all that jazz. The movie isn't meant to be realistic. It's meant to be exaggerated and cartoonish. So you can safely assume, in the context of the movie, Nygma had the means to create the fake security footage.
TedStixon